Lossless snubber circuit for use in power converters

ABSTRACT

A switching power converter employing a novel lossless zero-voltage-switching passive snubber network having a power range of up to 5 KW is presented. The passive snubber network improves efficiency, power density, and transient performance, reduces switching losses and EMI, and permits fixed frequency operation of the switching power converter. The passive snubber network also reduces and/or eliminates large peak currents and reverse recovery current spikes normally seen in conventional switching power converters. The proposed passive snubber network may be used in various switching power converter topologies such as boost, buck, balanced, and flyback power converters.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention concerns DC to DC and single phase fed AC to DC switching power converters, and in particular, concerns switching power converters, having an output power of up to 5 KW (Kilo Watts), employing a novel lossless zero-voltage-switching (ZVS) passive snubber network. The proposed passive snubber network improves efficiency, power density, and transient performance, reduces switching losses and electro-magnetic interference (EMI), and permits fixed frequency operation of switching power converters. The proposed passive snubber network also reduces and/or eliminates large peak currents and reverse recovery current spikes which normally occur in known switching power converters. The proposed passive snubber network may be used in various switching power converter topologies such as boost, buck, forward, and flyback power converters.

b) Prior Art

In the past, power conversion, such as DC to DC power conversion for example, was typically performed by hard switched, pulse width modulating (PWM) circuits such as the "boost" power converter shown in FIG. 1, for example. This known power converter includes a controllable switch S₁ (such as a transistor (e.g., a MOSFET), for example) which can be provided with a fixed frequency switching signal from a controller. By varying the duty cycle of the switching signal, the output current of the power converter is controlled. This is known as pulse width modulation (or PWM) control.

Specifically, the conventional boost power converter of FIG. 1 includes a series connection of an inductor L₁, a diode D₁, and a capacitor C₁ coupled with the input voltage V_(IN). The anode of the diode D₁ is coupled with the inductor L₁ while the cathode of the diode D₁ is coupled with the capacitor C₁. A load R_(L) to be supplied with an output voltage is coupled across the capacitor C₁. A controllable switch S₁, such as a MOSFET for example, includes a first terminal coupled with a node between the inductor L₁ and the anode of the diode D₁ and a second terminal coupled with a lower potential terminal of the input voltage source V_(IN).

In the conventional boost converter of FIG. 1, when the controllable switch S₁ is open (i.e., blocking), current flows through the inductor L₁ and the diode D₁. Since the uncharged capacitor C₁ initially appears as a short circuit (because it will draw current), the current flowing through the inductor L₁ and the diode D₁ will charge the capacitor C₁. When the controllable switch S₁ is subsequently closed (conducting), current from the input voltage V_(IN) will flow through the inductor L₁ and the controllable switch S₁ to ground (or to the negative terminal of the input voltage supply). Assuming that there are no losses in the inductor L₁, equating the volt-seconds across the inductor L₁ to zero, and ignoring the turn-on voltage of the diode D₁, the output voltage V_(OUT) can be determined from the following relationship: ##EQU1## where D is the duty cycle of the switching signal. Thus, assuming that a fixed frequency switching signal is provided to the controllable switch S₁, the higher the duty cycle of the switching signal, the higher the output voltage supplied across the output load R_(L).

The simple design of the known boost converter of FIG. 1 is based on an assumption that existing power switches closely approximate ideal switches, i.e., that the transitions from opened (i.e., blocking) to closed (i.e., conducting) and closed to opened occur instantaneously. Unfortunately, this assumption is not particularly accurate. Indeed, this assumption, and the power converter topologies it has spawned, are responsible for serious limitations in the performance of the conventional switching converters because the non-ideal (i.e., non-instantaneous) switching characteristic causes switching power (P=I_(switch) *V_(switch)) losses.

Specifically, one of the most important characteristics of a DC to DC power converter is that it has a high power density. To possess a high power density, the controllable switch of the power converter must operate at relatively high frequencies. Thus, conventional pulse width modulation power converters, such as the conventional boost power converter discussed above, are disadvantageous because increasing the switching frequencies to achieve higher power densities will cause an increase in switching losses.

In conventional hard switching PWM converters operating at frequencies below 100 KHz and at power levels up to 5 KW, diode stored charge, diode reverse recovery, and device switching losses are reportedly the biggest problems.

Diode reverse recovery in the conventional hard switching boost power factor converter presents a significant limitation because it generates substantial EMI and limits the power conversion frequency and efficiency, particularly in the 3 to 5 KW power range. Specifically, in the conventional hard switching boost converter of FIG. 1, when the switch S₁ is closed, the current through the switch S₁ increases to the level of the current through the inductor L₁. At this point, the current through the diode D₁ decreases until the diode D₁ no longer conducts. At this time, any charge stored on the diode D₁ is removed via switch S₁. As the charge is being removed from the diode D₁, the current through the switch S₁ continues to rise, often to a value of more than twice the inductor current level. The combination of high peak current, high dI/dt (current rate of change), and high dV/dt (voltage rate of change) when the voltage of the switch S₁ approaches the level of the lower potential terminal of V_(in) (or ground), creates significant unwanted RFI/EMI noise and considerably stresses the switch S₁.

The problems of conventional hard switching PWM power converters (such as boost, buck, forward, and flyback power converters) are explained in greater detail in the article D. M. Divan, "Soft Switching Converters: A Review," Soft Switching Converters: Topologies, Design, and Control: Summary of Publications 1986-1990: Wisconsin Electric Machines and Power Electronics Consortium, pp. 1-51 (1990) (hereinafter referred to as "the Divan article" and incorporated herein by reference).

In response to the problems associated with hard switching PWM power converters described above, designers have proposed the use of reactive snubber networks to divert energy that would be dissipated during switching transitions by "trapping" that energy, thereby permitting "soft-switching" of the controllable switch (i.e., switching when little or no voltage appears across the switch and/or when little or no current is flowing through the switch thereby reducing switching stresses). There are two types of such soft-switching converters: (1) zero current switching (ZCS) converters in which opening (switch-off) and closing (switch-on) of the controllable switch occur with no current in the controllable switch; and (ii) zero voltage switching (ZVS) converters in which opening (switch-off) and closing (switch-on) of the controllable switch occur with no voltage across the switch.

Zero current switching (ZCS) is accomplished generally by employing a purely inductive snubber. Zero voltage switching (ZVS) on the other hand is accomplished by employing a purely capacitive snubber having an anti-parallel diode. With ZVS snubber circuits, closing (switch-on) occurs only when the anti-parallel diode is conducting; opening (switch-off) losses decrease with increasing capacitance. Unfortunately, to cause conduction in the anti-parallel diode before turning on the switch, additional circuitry is required to discharge the capacitor or an external resistor is required to dissipate the energy stored in the capacitor during a turn-on part of the switching cycle. Thus, although the controllable switch can operate at elevated frequencies because switching stresses are reduced, the power losses are merely shifted from the controllable switch to the dissipating resistor. Furthermore, if a snubber circuit is not properly designed, it will present a low impedance to the switch when it is turned on and off (closed and opened) which results in a large current spikes.

To solve the power dissipation problem, designers have developed "lossless" soft switching power converters in which the snubber networks are reset by means of inherent circuit operation. These lossless soft switching power converters "recirculate" the energy stored by the reactive snubbers to accomplish lossless operation. Other "lossless" soft switching power converters have snubber networks which are reset by using additional auxiliary switches in conjunction with reactive elements. Unfortunately, this way of eliminating spikes requires extra power handling components. These additional power handing components add size, weight, and cost to the power conversion system. Moreover, they often severely reduce overall system efficiency since the RMS input current is high.

An example of a known zero voltage switching (ZVS) boost power converter (See e.g., J. Bazinet et al., "Analysis and Design of a Zero Voltage Transition Power Factor Correction Circuit," IEEE Applied Power Electronics Conference (APEC), pp. 591-597 (1994)) is illustrated in FIG. 2. As shown in FIG. 2, this known ZVS boost power converter is a modification of the hard switching boost converter of FIG. 1. Specifically, a capacitor C₂ is arranged across the switch S₁. A first series circuit, including an inductor L₂, a diode D₂ and a second switch S₂, is also arranged in parallel with the switch S₁. Further, a second series circuit, including a diode D₃, a diode D₄, and a resistor R₁ is arranged in parallel with the capacitor C₁. The first and second series circuits are electrically coupled, from a node between the inductor L₂ and the diode D₂ of the first series circuit to a node between the diodes D₃ and D₄ of the second series circuit.

The operation of the known ZVS boost power converter is explained below. To achieve zero voltage switching of the switch S₁, the auxiliary switch S₂ is turned-on (i.e., closed) near the end of the time switch S₁ is off (i.e., not conducting). Then, the current in the inductor L₂ increases until it reaches the level of the current in the input inductor L₁. Simultaneously, the capacitor C₂ and the inductor L₂ create a resonance thereby reducing the voltage across the switch S₁ to zero before the switch S₁ is turned-on (closed). The diode D₁ is turned off (opened) without the problem of a high reverse recovery current passing through the switch S₁. The capacitor C₂ minimizes the voltage across the switch S₁ to a very low value during turn-off (opening).

Unfortunately, this known ZVS boost power converter requires an active circuit element; namely, the second switch S₂. Being an active element, the second switch S₂ requires additional supporting circuitry (e.g., a base drive circuit) and introduces additional losses. The turn-off (opening) losses of the switch S₂ are significant because the inductor L₂ and the switch S₂ are carrying the load current before the switch S₂ is turned off (opened). Similarly, during the turn-on (closing) of the switch S₂, the capacitor C₂ will discharge through L₂ and S₂ thereby causing additional power dissipation. Therefore: (i) the energy stored in the parasitic capacitance of the switch S₂ dissipates in the switch S₂ during the turn-on (closing); (ii) the switch S₂ experiences substantial turn-off (opening) losses because before the turn-off (opening) of the switch S₂, it carries the full current of the inductor L₁ ; (iii) the inductor L₂ must be designed to limit any reverse recovery current spike from the diode D₁ during the turn-on (closing) of the switch S₂ ; (iv) the tailing effect of IGBTs (Insulated Gate Bipolar Transistors) during turn-off causes difficulties when using IGBTs in power converters having a power range of 3 to 5 KW.

Thus, the above mentioned known snubber circuits either: (a) use an active auxiliary switch in conjunction with the reactive elements (see e.g., the known ZVS boost power converter of FIG. 2) to relieve the voltage and current switching stresses of the controllable switch; or (b) use the controllable switch to provide energy to the circuit, that is, the energy used by the snubber circuit is drawn from the input and returned to the input.

Other known lossless power converters include resonant switching converters. These power converters incorporate reactive elements (capacitors and inductors) in conjunction with the switching device. The output voltage of these circuits is controlled by varying the operating frequency of the controllable switches. These circuits advantageously have low semiconductor switching losses and operate with sinusoidal waveforms. Unfortunately, resonant power converters exhibit increased component count, increased switching currents (peak and RMS) and require wide operating frequency variations to maintain a constant output voltage. Thus, resonant switching converters are relatively expensive, require relatively complex switching control circuitry, and eliminate switching losses at the expense of conducting losses.

A further example of a zero voltage switching (ZVS) quasi-resonant boost power converter is illustrated on page 14 of the Divan article. In this zero voltage switching (ZVS) boost power converter, a capacitor, along with an anti-parallel diode, are coupled across the controllable switch S₁ and an inductor is placed in series with the controllable switch S₁. (See phantom lines in FIG. 1.) The operation of this power converter is explained below. During the turn-off of the switch S₁, the inductor L₂ and the capacitor C₂ cause a resonance, thereby making the voltage across the switch S₁ nearly zero. Moreover, the switch S₁ is closed when its anti-parallel diode D₂ is conducting. Thus, the current is zero during turn-on (closing) of the switch S₁. However, the current flowing through the switch S₁ is sinusoidal. Thus, the peak and RMS (root mean square) currents are increased. Consequently, as stated above, the quasi-resonant power converter eliminates switching losses at the expense of conduction losses. Furthermore, a wide range of operating switching frequencies is required to maintain a constant output voltage.

Therefore, although these known lossless boost power converters have better characteristics than the hard switching converters and non-lossless soft-switching boost power converters, certain disadvantages remain. Specifically, the known ZVS boost power converter requires an additional auxiliary switch which requires additional support circuitry (e.g., a base drive circuit) and which introduces additional parasitic losses. Furthermore, the turn-off (opening) of the additional switch is not lossless as explained earlier. On the other hand, although resonant switching power converters eliminate switching losses, they introduce conducting losses. Moreover, as stated above, the output voltage in the resonant switching converters is controlled by frequency modulation. Therefore, this circuit requires additional circuitry to carry out such frequency modulation. Accordingly, an improved switching power converter employing an improved snubber network is needed.

The improved snubber network used by such a switching power converter should be a passive network; that is, the snubber network should include only resistors (not required in the present invention), capacitors, inductors, and diodes. The improved snubber network should use relatively inexpensive components. Moreover, the improved snubber network should enable the switching power converter to operate at higher switching frequencies to increase power density and to permit smaller components to be used. The improved snubber network should also eliminate reverse recovery current spikes in the switching power converter. The improved lossless soft switching power converter using the improved snubber network should also limit peak device voltage and current stresses, limit peak capacitor voltages, limit RMS currents in all components, have low sensitivity to second order effects (particularly when high switching frequencies are used), not require complex control circuitry, be fault tolerant, and have low electro-magnetic interference (EMI) and low radio frequency interference (FI). Lastly, the improved snubber network should be adaptable for use in various switching power converter topologies such as boost, buck, forward, and flyback power converters.

SUMMARY OF THE INVENTION

The present invention provides an improved lossless soft switching power converter having no additional active components and having relatively low conduction losses compared to resonant topologies. The present invention does so by modifying conventional switching power converters by providing an improved snubber network.

The snubber network of the present invention is used with a device for converting power from an input voltage source to be supplied to an output load via a first inductor and a first diode, based on a duty cycle of a switching signal supplied to a controllable switch. The snubber network of the present invention includes three series connections. The first series connection includes an inductor, a diode, and a capacitor, the second series connection includes a capacitor and a diode, and the third series connection includes a diode and an inductor. The third series connection is arranged between the first and second series connection such that the inductor of the third series connection is coupled at a node between the diode and the capacitor of the first series connection and the diode of the third series connection is coupled at a node between the capacitor and the diode of the second series connection. More specifically, the inductor of the third series connection is coupled with the cathode of the diode of the first series connection and the cathode of the diode of the third series connection is coupled with the anode of the diode of the second series connection.

The snubber network of the present invention is incorporated into a power converter as follows. The first series connection is coupled across the controllable switch of the power conversion circuit, the second series connection is arranged such that the cathode of the diode of the second series connection is coupled with the output voltage appearing across the output load (or alternatively, with the input voltage), and the third series connection is arranged between the first and second series connection such that the inductor of the third series connection is coupled at a node between the diode and the capacitor of the first series connection and the diode of the third series connection is coupled at a node between the capacitor and the diode of the second series connection.

In a preferred embodiment of the snubber network, a further inductor is coupled with the cathode of the diode of the power conversion circuit.

The snubber network of the present invention is designed for use with a switching power converter such that when the controllable switch is closed, voltage stored in the capacitor of the second series connection increases to the output voltage and when the controllable switch is opened, the voltage stored in the capacitor of the second series connection decreases to zero. Further, when the controllable switch is closed, voltage stored in the capacitor of the first series connection decreases from the output voltage to zero and when the controllable switch is opened, the voltage stored in the capacitor of the first series connection increases to the output voltage. The voltage of the first series connection may slightly overshoot the output voltage.

The snubber network of the present invention is designed for use with a switching power converter such that, at the instant when the controllable switch is opened, current through the controllable switch is relatively low such that switching losses are minimized. Further, at the instant when the controllable switch is closed, voltage across the controllable switch is relatively low such that switching losses are minimized.

In an alternative embodiment, the snubber network may also include a clamping diode coupled across the capacitor of the first series connection.

The snubber network of the present invention may be used with various switching power converter topologies such as boost power converters, buck power converters, forward power converters, and flyback power converters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic that illustrates a conventional hard switching PWM power converter having a boost topology.

FIG. 2 is a schematic that illustrates a conventional ZVS soft switching boost power converter.

FIG. 3 is a schematic that illustrates the improved lossless soft switching PWM boost power converter employing the improved snubber network of the present invention and illustrates the state of the power converter at an instant just before the switch is turned-on (closed).

FIG. 4a is a schematic of the power converter of FIG. 3 that illustrates the operation of the power converter when the switch is closed. FIG. 4b is a schematic of the power converter of FIG. 3 which illustrates the initial condition of capacitor C₂ at an instant just before the switch is turned-off (opened). FIG. 4c is a schematic of the power converter of FIG. 3 that illustrates the operation of the power converter when the switch is open.

FIGS. 5a through 5c are timing diagrams illustrating the zero voltage turn-off and zero current turn-on of the switch of the improved lossless soft switching PWM boost converter of the present invention.

FIGS. 6a through 6i are timing diagrams illustrating the currents and voltages across various elements of the improved lossless soft switching PWM boost converter of the present invention.

FIG. 7 is a schematic of an alternative embodiment of a lossless boost power converter using the improved snubber network of the present invention.

FIG. 8 is a schematic of a lossless buck power converter using the improved snubber network of the present invention.

FIG. 9 is a schematic of an alternative embodiment of the lossless buck power converter using the improved snubber network of the present invention.

FIG. 10 is a schematic of a further alternative embodiment of the lossless buck power converter using the improved snubber network of the present invention.

FIG. 11 is a schematic of a lossless forward power converter using the improved snubber network of the present invention.

FIG. 12 is a schematic of a lossless flyback power converter using the improved snubber network of the present invention.

DETAILED DESCRIPTION

FIG. 3 is a schematic that illustrates a lossless soft switching PWM boost power converter which uses the improved passive snubber network of the present invention. As shown in FIG. 3, as with the known hard-switching power converter, the improved power converter of the present invention includes a voltage source V_(in), an inductor L₁, a diode D₁, and an output load R_(L), as well as a switch S₁ arranged in parallel with the current fed source network of V_(in) and L₁.

The improved passive snubber network of the present invention used with this boost power converter, provides a first series circuit, having a saturable inductor L₃, a diode D₂ and a capacitor C₃, in parallel with the switch S₁. The first series circuit has a first terminal coupled with a node between the inductor L₁, the switch S₁, the anode of the diode D₁ and capacitor C₂ and a second terminal coupled with the lower potential terminal of the input voltage source V_(in). Thus, the first series circuit of the improved snubber network is arranged across the switch S₁. Within the first series circuit, the inductor L₃ is coupled with the first terminal and the capacitor C₃ includes a first side coupled with the cathode of the diode D₂ and a second side coupled with the second terminal.

The improved snubber network of the present invention used with the boost power converter further provides a second series circuit, having capacitor C₂ and a diode D₃, in parallel with the diode D₁ and the inductor L₄. Within the second series circuit, the capacitor C₂ has a first side coupled with the anode of the diode D₃ and a second side coupled with the anode of the diode D₁. Thus, the second series circuit of the improved snubber network is clamped to the output voltage appearing across the output load R_(load).

The improved snubber network of the present invention used with the boost power converter also provides a third series circuit having an inductor L₂ and a diode D₄. Within the third series circuit, a first terminal of the inductor L₂ is coupled at a node between the cathode of the diode D₂ and the capacitor C₃ of the first series circuit, a second terminal of the inductor L₂ is coupled with the anode of the diode D₄, and the cathode of the diode D₄ is coupled at a node between the capacitor C₂ and the anode of the diode D₃ of the second series circuit.

The improved snubber network of the present invention used with the boost power converter may also include an inductor L₄ coupled with the cathode of the diode D₁. This inductor L₄ is used to reduce current spikes otherwise caused by the reverse recovery of the diode D₁. Thus, the inductor L₄ helps reduce EMI and RF emission.

In the circuit of FIG. 3, the capacitance of the capacitor C₂ (which may have a value on the order of pico-farads) is much less than the capacitance of the capacitor C₁ (which may be on the order of micro-farads).

The overall operation of the boost power converter using the improved snubber network of the present invention is similar to the hard switching boost power converter of FIG. 1. The operation of the snubber network of the present invention when used in a boost power converter is explained below with reference to FIGS. 3, 4a through 4c, 5a through 5c, and 6a through 6I.

The state of the boost power converter of FIG. 3 at a time when the switch S₁ is turned off (not conducting), just before the switch S₁ is turned on, is as follows. Current flows to the output load via the inductor L₁, the diode D₁, and the inductor L₄. Current flowing through the loop defined by the inductor L₁, the capacitor C₂, the diode D₃, the load R_(L), and the voltage supply charges the capacitor C₂ to less than one volt (i.e., a diode forward drop) (See FIG. 6f) and the capacitor C₃ to V_(out) (See FIG. 6h).

As shown in FIG. 4a, at a first instant in time, when the switch S₁ is first closed, a current will flow through a loop defined by the capacitor C₃, the inductor L₂, the diode D₄, the capacitor C₂, and the switch S₁, thereby discharging the capacitor C₃ as shown in FIG. 6h and charging the capacitor C₂ as shown in FIG. 6f. This continues until the reversed charge on the capacitor C₂ reaches V_(out). The value of the capacitor C₃ is selected such that when the charge on the capacitor C₂ reaches V_(out), the charge on the capacitor C₃ will be zero. (See times t₁ and t₂ of FIGS. 6f and 6h). The charge on the capacitor C₂ and the discharging of the capacitor C₃ may occur about less than 1 (one) μs after the switch S₁ is closed. If the voltage of the capacitor C₂ reaches V_(out) (plus the turn-on voltage of the diode D₃), the diode D₃ becomes forward biased thereby clamping the voltage of the capacitor C₂ to output voltage.

The current (I_(C2)) flowing through the loop defined by the capacitor C₃, the inductor L₂, the diode D₄, the capacitor C₂ and the switch S₁ can be expressed by the following: ##EQU2## where the power semiconductor forward voltage drops are neglected, and where 0<t<t₁.

This expression can be reduced to the following: ##EQU3## wherein 0<t<t₁.

A solution of this differential equation is given by:

    I.sub.C2 (ωt)=A sin(ωt)+B cos(ωt)        (5)

where ##EQU4## where 0<t<t₁.

At time t=0, the switch S₁ is turned-on (closed) and I_(C2) (0)=0. Therefore, 0=A sin 0+B cos 0 and B=0. Accordingly,

    I.sub.C2 (ωt)=A sin(ωt).                       (6)

The voltage of the capacitor C₃ is expressed as: ##EQU5##

At time t=0, V_(C3) =V_(OUT). Therefore: ##EQU6##

Therefore, as shown in FIG. 6c, the current I_(C2) can be expressed as:

    I.sub.C2 (ωt)=A sin(ωt)+B cos(ωt)        (10)

    =ωC.sub.3 V.sub.OUT sin ωt,                    (11)

where 0<t<t₁.

The operation of the DC to DC power converter is now explained with reference to FIG. 6b. Under steady state operating conditions, when the switch S₁ is turned-on (closed), the current in the inductor L₁ rises at a rate of _(L).sbsb.1^(V).sbsp.in and when the switch S₁ is turned-off (opened), the current falls at a rate of ##EQU7## The input current ripple can be controlled by controlling the frequency of the switching signal and the output power can be controlled by controlling the duty cycle of the switching signal. Specifically, the duty cycle is related to the input and output voltages from equation (1): ##EQU8##

V_(IN) is a DC voltage source in the case of a DC to DC power converter and V_(IN) =|V* sin(wt)| in the case of an AC to DC power converter. Assuming a known input current in the case of a DC to DC power converter, the current I_(S1) through the switch S₁ is the sum of the input current I_(i) times the duty cycle D, and the snubber current I_(C2) derived above and illustrated in FIG. 6d. In other words: I_(S1) (ωt)=I_(i) *D+I_(C2) (where 0≦ωt≦t_(on)). The current I_(S1) through switch S₁ during turn-on is illustrated in FIG. 6d.

Further, when the switch S₁ is closed, the current I_(S1) through the switch S₁ will not rise instantaneously because the inductor L₄ will continue to conduct a decaying current as shown in FIG. 5c. Therefore, after the switch S₁ is turned-on (closed), the current I_(S1) through the switch S₁ will rise linearly as the current through the inductor L₄ decays linearly. (See FIGS. 5a and 5c.) This linear decay facilitates zero-current turning-on (closing) of the switch S₁. Furthermore, the decaying current in the inductor L₄ causes the current through the diode D₁ to go to zero. The diode D₁ then turns-off. Thus, the current spike caused by diode reverse recovery associated with the known boost power converter design is eliminated.

As discussed above, after the switch S₁ is turned-on, the voltage of the capacitor C₂ reaches V_(out) and the voltage on the capacitor C₃ is zero, the current continuing to flow through the inductor L₂ decays to zero. That is, the energy stored in the inductor L₂ is given to the load R_(L) via diodes D₄ and D₃. The decay of the current in the inductor L₂ occurs quickly because the voltage of the capacitor C₃ is much less than that of the capacitors C₁ and C₂.

Consequently, when the switch S₁ is turned-on (closed), the current flowing through it I_(S1) (See FIG. 6d) is a combination of: (i) the current flowing through the inductor L₁ (See FIG. 6b); and (ii) the current flowing from the capacitor C₃ to the capacitor C₂ via the inductor L₂ and the diode D₄ (See FIG. 6c).

When the switch S₁ is turned-on (closed), current also flows through the input voltage supply V_(in), the inductor L₁ and the switch S₁.

Thus, the state of the power converter of FIG. 4b at a time when the switch S₁ is on (conducting), just before the switch S₁ is turned off, is as follows. The voltage across the capacitor C₃ is zero (See FIG. 6h) and the voltage across the capacitor C₂ is V_(out) (See FIG. 6f). Since the charge on the capacitor C₂ is V_(out), there is no voltage drop across the switch S₁.

At this time, the switch S₁ is turned-off (opened) as shown in FIG. 4c and FIGS. 5a through 5c and 6a through 6i. As FIG. 4c further illustrates, the current of the inductor L₁ flows through: (i) a loop defined by the capacitor C₂, the diode D₃, the capacitor C₁, and the voltage source V_(in) (see FIG. 6c); and (ii) a loop defined by the diode D₂, the capacitor C₃, and the voltage supply V_(in). Therefore, the voltage rise of the capacitor C₃ depends on the value of the current flowing through L₁ before the switch S₁ is turned-off and the capacitance of the capacitor C₃. The voltage of the capacitor C₂ is discharged at a rate determined by the value of the current flowing through the inductor L₁ just before the switch S₁ is turned-off and the capacitance of the capacitor C₂.

Assuming that the capacitors C₂ and C₃ have the same capacitance, the design of the circuit can be simplified. For example, if the capacitors C₂ and C₃ have the same capacitance, the current flowing through the capacitor C₂ and the capacitor C₃ is half the value of the current I_(L1) flowing through the inductor L₁ before the switch S₁ is turned-off. Therefore, the voltage rise across the switch S₁ during turn-off can be expressed as:

    dV.sub.S1 /dt=I.sub.L1 /2C.sub.3                           (13).

Using equation (13), the value of the capacitor C₃ for any given switch S₁ (at turn-off time) can be determined. During turn-off of the switch S₁, the current through the switch S₁ starts decreasing and the magnitude of the current through the capacitors C₂ and C₃ starts increasing as shown in FIGS. 6c, 6d, and 6i. Thus, the current I_(L1) through the inductor L₁ can be expressed as:

    I.sub.L1 (t)=I.sub.S1 (t)+I.sub.C2 (t)+I.sub.C3 (t) where T2<t<T3. (14)

As shown in FIG. 5a, the voltage across the switch S₁ will rise from zero to the voltage of the capacitor C₁ plus a small voltage (due to inductor L₄) until the diode D₁ becomes forward biased. The value of this switch voltage overshoot during turn-off is dictated by the inductance of the inductor L₄. However, if the snubber network is designed properly, the overshoot will be limited to 60 to 70 volts higher than the output voltage V_(out). This overshoot is not a problem in a boost power converter in the range of 3 to 5 KW. For example, a 4 KW power factor converter has been implemented using a 500 V MOSFET switch. In this converter, the peak voltage (including overshoot) of the switch was found to be 450 V. Thus, in such an implementation, the overshoot is not a problem Once the diode D₁ starts conducting, the current flowing through the inductor L₁ begins to flow through the diode D₁. These conditions remain the same until the next turn-on (closing) of the switch S₁.

As described above with respect to a boost power converter topology, an improved snubber network in accordance with the present invention is provided. The improved snubber network comprises: (i) a first series connection of the inductor L₃, the diode D₂, and the capacitor C₃ coupled across the switch S₁ ; (ii) a second series connection of the capacitor C₂ and the diode D₃ arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the output voltage appearing across the load R_(L) ; (iii) a third series connection of the diode D₄ and the inductor L₂ having a first end terminal coupled at a node between the diode D₂ and the capacitor C₃ of the first series connection and having a second end terminal coupled at a node between the capacitor C₂ and the diode D₃ of the second series connection; and (iv) the inductor L₄ coupled with the cathode of the diode D₁. To reiterate, this improved snubber network uses only passive circuit elements.

The improved snubber network of the present invention can be used in other power converter topologies as well. In each case, the first series connection of the inductor L₃, the diode D₂, and the capacitor C₃ is coupled across the switch S₁ ; the second series connection of the capacitor C₂ and the diode D₃ is arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the input voltage source V_(in) or with the output load R_(out) ; the third series connection is coupled between the first and second series circuit; and the inductor L₄ is coupled with the diode D₁.

For example, the improved snubber network of the present invention can be used in the alternative boost power converter shown in FIG. 7. The alternative embodiment of the boost power converter shown in FIG. 7 differs from the embodiment shown in FIG. 3 in that: (i) the cathode of the diode D₃ is coupled with the input voltage source V_(in) rather than the output load R_(load) ; (ii) an additional capacitor C₄ is arranged across the input voltage source V_(in) ; and (iii) an additional diode D₅ is provided across the capacitor C₃.

The operation of the alternative boost power converter of FIG. 7 is almost the same as the boost power converter of FIG. 3 except that: (i) the voltage stored by the capacitor C₂ is clamped to the input voltage and therefore changes between +V_(in) and -V_(in) during the turn-off of the switch S₁ ; and (ii) the diode D₅ prevents the capacitor C₃ from becoming negatively charged.

The improved snubber network can also be used in other power converter topologies. For example, FIGS. 8 through 10 are schematics showing the snubber network used in three embodiments of a buck converter topology. As shown in FIG. 1.1(a) on page 4 of the Divan article, a buck converter generally includes a series connection of an input voltage supply V_(in) (shown as V₁ in the Divan article), a controllable switch S₁, an inductor L₁, and an output load R_(load). A diode D₁ (shown as S2 in the Divan article) is arranged across the input voltage supply V_(in) such that its anode is coupled with a lower potential terminal of the input voltage supply V_(in) and its cathode is coupled at a node between the controllable switch S₁ and the inductor L₁. An output capacitor C₁ is arranged across the output load R_(load).

FIG. 8 is a schematic which shows a first embodiment of the buck power converter using the improved snubber network of the present invention in which (i) the first series circuit of the inductor L₃, the diode D₂ and the capacitor C₃ is arranged across the controllable switch S₁ ; (ii) the second series circuit of the capacitor C₂ and the diode D₃ is arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the output load R_(load) (alternatively, the cathode of the diode D₃ may be coupled with the input voltage supply V_(in)); (iii) the third series circuit of the diode D₄ and the inductor L₂ is arranged between the first and second series circuits; and (iv) the inductor L₄ is coupled with the anode of the diode D₁. The voltage of the capacitor C₂ is clamped to V_(out) during turn-on of the switch S₁ and charges to +V_(in) and -V_(in) during turn-off of the switch S₁. Similar to the boost power converter of FIG. 7, a diode D₅ is provided across the capacitor C₃ to prevent the capacitor C₃ from becoming negatively charged.

FIG. 9 is a schematic which shows a second embodiment of a buck power converter using the improved snubber network of the present invention, in which: (i) the first series circuit of the inductor L₃, the diode D₂ and the capacitor C₃ is arranged across the controllable switch S₁ ; (ii) the second series circuit of the capacitor C₂ and the diode D₃ is arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the output load R_(load) (alternatively, the cathode of the diode D₃ may be coupled with the input voltage supply V_(in) ; (iii) the third series circuit of the diode D₄ and the inductor L₂ is arranged between the first and second series circuits; and (iv) the inductor L₄ is coupled with the anode of the diode D₁. In this second embodiment of the buck power converter, the voltage of the capacitor C₂ is clamped to the voltage across the output load R_(load). A diode D₅ may be coupled across the capacitor C₃ to prevent it from becoming negatively charged.

FIG. 10 shows a third embodiment of a buck power converter using the improved snubber network of the present invention, in which: (i) the first series circuit of the inductor L₃, the diode D₂ and the capacitor C₃ is arranged across the controllable switch S₁ ; (ii) the second series circuit of the capacitor C₂ and the diode D₃ is arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the input voltage source V_(in) (alternatively, the cathode of the diode D₃ may be coupled with the output load R_(load)); (iii) the third series circuit of the diode D₄ and the inductor L₂ is coupled between the first and second series circuits; and (iv) the inductor L₄ is coupled with the anode of the diode D₁. An additional diode D₅ may be coupled across the capacitor C₃ to prevent the capacitor C₃ from becoming negatively charged.

FIG. 11 is a schematic which shows a forward power converter using the improved snubber network of the present invention. A conventional forward power converter is illustrated in FIG. 1.2(b) on page 4 of the Divan article. In the conventional forward power converter, a first loop is formed from a series connection of an input voltage source V_(in) (labeled V₁ in the Divan article), a primary winding of a transformer, and a controllable switch S₁. A second loop includes a series connection of a diode D₅ (labeled S3 in the Divan article), the output load R_(load) (labeled as Z_(L) in the Divan article), the inductor L₁, and the secondary of the transformer. An output capacitor C₁, as well as a diode D₁ (labeled as S2 in the Divan article), are arranged across the output load R_(load).

In the forward power converter including the improved snubber network of the present invention of FIG. 11: (i) the first series circuit of the inductor L₃, the diode D₂ and the capacitor C₃ is arranged across the controllable switch S₁ ; (ii) the second series circuit of the capacitor C₂ and the diode D₃ is arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the input voltage source V_(in) ; (iii) the third series circuit of the diode D₄ and the inductor L₂ is coupled between the first and second series circuits; and (iv) the inductor L₄ is coupled with the anode of the diode D₁. In this forward power converter, the improved snubber network of the present invention clamps the voltage of the capacitor C₂ to V_(in) during the turn-on period of the switch S₁. This circuit advantageously permits the PWM control to be used for output voltage control. A further diode D₆ may be arranged across the capacitor C₃ to prevent the capacitor C₃ from becoming negatively charged.

FIG. 12 is a schematic of a flyback power converter using the improved snubber network of the present invention. A conventional flyback power converter is shown in FIG. 1.2(a) of the Divan article. The conventional flyback power converter includes a first loop including a series connection of the input power supply V_(in) (labeled as V₁ in the Divan article), a primary winding of a transformer, and a controllable switch S₁. The conventional flyback power converter further includes a second loop including a diode D₅ (labeled as S2 in the Divan article) an output load R_(load) (labeled as Z₁ in the Divan article), an inductor L₁, and a secondary winding of the transformer. An output capacitor C₁ is arranged across the output load R_(load), as is a diode D₁.

The flyback power converter of FIG. 12, which includes the improved snubber network, includes: (i) the first series circuit of the inductor L₃, the diode D₂ and the capacitor C₃ arranged across the controllable switch S₁ ; (ii) the second series circuit of the capacitor C₂ and the diode D₃ arranged such that the capacitor C₂ is coupled with a terminal of the switch S₁ and the cathode of the diode D₃ is coupled with the input voltage source V_(in) ; (iii) the third series circuit of the diode D₄ and the inductor L₂ coupled between the first and second series circuits; and (iv) the inductor L₄ is coupled with the anode of the diode D₁. In the flyback power converter of FIG. 12, the improved snubber circuit clamps the voltage of the capacitor C₂ to V_(in) during the switch-on period of the controllable switch S₁ and allows zero current turn-on of the switch S₁.

As described above with respect to the first embodiment of the boost power converter (See FIG. 3), when the switch S₁ is turned-off (opened), no voltage exists across it (See FIG. 5a), and when the switch S₁ is turned-on (closed), no current instantaneously flows through it (See FIG. 5c). Accordingly, the switch operation is lossless. Moreover, operation of the power converters using the snubber network of the present invention at high switching frequencies is practical. Since the operation of the improved power converters using the snubber network of the present invention is practical at high switching frequencies, the power density of the power converter can be increased and smaller components can be used. Further, the lossless switching of the improved power converters using the snubber network of the present invention limits peak device voltage and current stresses, limits peak capacitor voltages, limits RMS currents in all components, is relatively insensitive to second order effects (particularly when high switching frequencies are used), and has relatively low electro-magnetic interference (EMI) and low radio frequency interference (RFI). Moreover, the improved power converters using the snubber network of the present invention reduces conducting losses and is operable with a fixed frequency switching signal.

Furthermore, the lossless operation of the improved power converters of the present invention is achieved with a snubber circuit employing only passive circuit elements. That is, the snubber circuit in the improved power converter of the present invention includes only capacitors, inductors, and diodes. Since the improved power converters of the present invention does not require any additional active elements, such as a transistor switch for example, the number of relatively expensive components is reduced.

Lastly, the improved converters using the snubber network of the present invention eliminate reverse recovery spikes because the decaying current through the inductor L₄ eliminates the diode reverse recovery of the diode D₁. 

What is claimed is:
 1. A snubber network for use with a device for converting power from an input voltage source to be supplied to an output load via a first inductor and a first diode, based on a duty cycle of a switching signal supplied to a controllable switch, the snubber network comprising:a) a first series connection of an inductor, a diode, and a capacitor; b) a second series connection of a capacitor and a diode; and c) a third series connection of a diode and an inductor, the third series connection arranged between the first and second series connection such that the inductor of the third series connection is coupled at a node between the diode and the capacitor of the first series connection and the diode of the third series connection is coupled at a node between the capacitor and the diode of the second series connection.
 2. The snubber network of claim 1 wherein the inductor of the third series connection is coupled with the cathode of the diode of the first series connection and wherein the cathode of the diode of the third series connection is coupled with the anode of the diode of the second series connection.
 3. In a power conversion circuit which converts an input voltage to an output voltage supplied across a load based on a duty cycle of a switching signal supplied to a controllable switch, and which includes an inductor and a diode arranged in a main current path and a capacitor arranged across the load, a snubber network comprising:a) a first series connection of an inductor, a diode, and a capacitor, the first series connection being coupled across the controllable switch of the power conversion circuit; b) a second series connection of a capacitor and a diode, the second series connection being arranged such that the cathode of the diode of the second series connection is coupled with the output voltage; and c) a third series connection of a diode and an inductor, the third series connection arranged between the first and second series connection such that the inductor of the third series connection is coupled at a node between the diode and the capacitor of the first series connection and the diode of the third series connection is coupled at a node between the capacitor and the diode of the second series connection.
 4. The snubber network of claim 3 further comprising a further inductor coupled with the cathode of the diode of the power conversion circuit.
 5. The snubber network of claim 3 wherein when the controllable switch is closed, voltage stored in the capacitor of the second series connection increases to the output voltage and when the controllable switch is opened, the voltage stored in the capacitor of the second series connection decreases to zero.
 6. The snubber network of claim 5 wherein when the controllable switch is closed, voltage stored in the capacitor of the first series connection decreases from the output voltage to zero and when the controllable switch is opened, the voltage stored in the capacitor of the first series connection quickly increases to the output voltage.
 7. The snubber network of claim 6 wherein the voltage of the first series connection overshoots the output voltage.
 8. The snubber network of claim 3 wherein, at the instant when the controllable switch is opened, current through the controllable switch is relatively low such that switching losses are minimized.
 9. The snubber network of claim 3 wherein, at the instant when the controllable switch is closed, voltage across the controllable switch is relatively low such that switching losses are minimized.
 10. The snubber network of claim 3 further comprising a clamping diode coupled across the capacitor of the first series connection.
 11. The snubber network of claim 2 wherein the device for converting is a boost power converter.
 12. The snubber network of claim 2 wherein the device for converting is a buck power converter.
 13. The snubber network of claim 2 wherein the device for converting is a forward power converter.
 14. The snubber network of claim 2 wherein the device for converting is a flyback power converter.
 15. In a power conversion circuit which converts an input voltage to an output voltage supplied across a load based on a duty cycle of a switching signal supplied to a controllable switch, and which includes an inductor and a diode arranged in a main current path and a capacitor arranged across the load, a snubber network comprising:a) a first series connection of an inductor, a diode, and a capacitor, the first series connection being coupled across the controllable switch of the power conversion circuit; b) a second series connection of a capacitor and a diode, the second series connection being arranged such that the cathode of the diode of the second series connection is coupled with the input voltage; and c) a third series connection of a diode and an inductor, the third series connection arranged between the first and second series connection such that the inductor of the third series connection is coupled at a node between the diode and the capacitor of the first series connection and the diode of the third series connection is coupled at a node between the capacitor and the diode of the second series connection.
 16. The snubber network of claim 15 further comprising a further inductor coupled with the cathode of the diode of the power conversion circuit.
 17. The snubber network of claim 15 wherein when the controllable switch is closed, voltage stored in the capacitor of the second series connection increases to the output voltage and when the controllable switch is opened, the voltage stored in the capacitor of the second series connection decreases to zero.
 18. The snubber network of claim 13 wherein when the controllable switch is closed, voltage stored in the capacitor of the first series connection decreases from the output voltage to zero and when the controllable switch is opened, the voltage stored in the capacitor of the first series connection quickly increases to the output voltage.
 19. The snubber network of claim 18 wherein the voltage of the first series connection overshoots the output voltage.
 20. The snubber network of claim 15 wherein, at the instant when the controllable switch is opened, current through the controllable switch is relatively low such that switching losses are minimized.
 21. The snubber network of claim 15 wherein, at the instant when the controllable switch is closed, voltage across the controllable switch is relatively low such that switching losses are minimized.
 22. The snubber network of claim 15 further comprising a clamping diode coupled across the capacitor of the first series connection. 